Both 姿 (シ) and 勢 (セイ) are read with their on'yomi. There is no voicing or other sound changes like しぜい. The long vowel for セイ is important; しせ is a common pronunciation error. It refers to one's physical bearing or mental attitude towards a situation.

姿勢は姿(シ)と勢(セイ)の音読みです。濁音化しないため「しぜい」は誤りです。体の構え方の他、物事に対する考え方や態度も意味します。